Merge 'Add simple version of activity masking.'

Merge commit 'refs/changes/79/779/2' of
    https://review.webmproject.org/p/libvpx

Conflicts:
	vp8/encoder/encodeintra.c
	vp8/encoder/encodemb.c

Change-Id: Id607063fabe92d99eeb3c380e8ca670b01bfb3ef